Once you gave over the guild leadership to BL, you lost any right to the guild in any fashion, unfortunately. Within the rules of the game and all of that, they didn’t need to ask for permission since the guild became theirs to do with it what they wished, as Kozzae pointed out. Morally (I suppose?), they should have asked if you all kept in touch after you left the game, but that holds no weight in the black and white of what guild leadership is here in the game. Your GM passed the leadership. With that, they passed on any rights in the decision-making for the guild itself.
I can understand the sentiment and I empathize with you guys, but I say what I say as how things are dictated through the rules of guild management - not to be cruel, just the cut and dry of the rules.
Was the original GM on a different account at the time they were GM? People transfer characters and bounce around accounts sometimes, perhaps that could play a role in why they cannot seem to be verifying the OG GM as the original?
And too, again from the black and white perspective thing I keep hinging on - this article. I know it wasn’t a dethroning, but still.